Guy dates girl. Girl gets in a car accident and is left in a coma, the girl's best friend eventually gets guy to get over his own misery then starts going out with him herself. Girl finally wakes up from coma after three years- Drama then ensures. Has a sequel of sorts in the form of the Muv-Luv series.
Bittersweet Ending: (Mitsuki's route, which was followed by the TV adaptation): In the end, Haruka decides she can't be with Takayuki because he's not the same person he was three years ago; Takayuki finds Mitsuki and convinces her not to leave, and says he won't see Haruka again. It then skips to the first children's book Haruka wrote, about a character who just wants to get to the top of a hill to see her friends once more. The last shot is of Haruka standing in front of the tree, hopefully waiting.
And even when they're together, Takayuki has foregone his chance for a better full-time job, while Mitsuki not only couldn't get back to her swimming career she abandoned long ago, she also lost her Office Lady job. But at least they're together.
The OVA "second season" which follows a Haruka route ends with Takayuki and Haruka renewing their relationship and love for one another in the same spot where her traumatic accident occurs and in an almost sickeningly sweet Tastes Like Diabetes moment of all the onlookers clapping on for them. However, Mitsuki, who has almost been Demoted to Extra in this continuity, gave up her swimming career and didn't even get Takayuki either. But she and Haruka maintain their friendship as she willingly bows out of the Love Triangle.
The Cameo/Marth Debuted in Smash Bros.: The adaptation has the first animated appearances of Yayoi Isumi (Takayuki's landlady, originally from age's debut title Kimi Ga Ita Kisetsu) and Chizuru Sakaki (Akane's rival and best friend, originally from the quasi-sequel Muv-Luv).
Deconstruction : Of the very concept of True Companions by showing just how impermanent the bonds of childhood/ adolescent friendship truly are, especially in the face of major, life altering experiences. It also shows how trying to keep up the pretense after the bonds fade can only make things worse.
Demoted to Extra: If you only see the anime, you wouldn't notice that some extras like the doctor or some of her assistants, and Mayu/Ayu can be love interests.
Evolving Credits: The series doesn't have the normal opening until the third episode, when the main story takes place.
Half Truth: Everyone with respect to Haruka after she wakes up.
Heroic BSOD: Takayuki, after the accident. He stays that way until Mitsuki finally snaps him out of it.
I Coulda Been a Contender: Hayase Mitsuki, a once-in-a-generation swimming prodigy, has the makings of an Olympic-Grade Champion. She leaves it behind to take care of her best friend Haruka's boyfriend Takayuki, so suicidally depressed over Haruka's car-accident induced coma (which Mitsuki blames herself for) that he could not finish high school, and ends up with Takayuki herself. Three years later when Haruka awakens, Takayuki flirts on the edge of leaving Mitsuki. Adding insult to injury, Mitsuki (now a mere office lady) meets an old rival who is now an Olympic-level champion swimmer herself, doubly reminding her of the meaninglessness of her sacrifice. And to make matters worse, once she reveals what she did to the awakened Haruka, Haruka did NOT take it well and drove her away, destroying their friendship and really leaving Mitsuki with literally nothing.
Also Akane. Did her parents even bother to tell her that "Takayuki won't be seeing her anymore, since we told him to stop coming"? Nope, hence the incredibly painful to watch scene where Akane tells both of them off for not coming to visit (in the latter case, Mitsuki actually wanted to visit but was called a traitor by Akane herself).
Love Triangle: Between the newly awakened Haruka, Mitsuki, and Takayuki.
Omake: Instead of a next episode preview, there's an omake at the end of each episode.
One Side of the Story: Akane, when she walks in on Mitsuki in Takayuki's bed - without knowing that her parents had just told him to leave, or that Mitsuki offering herself to him might have been the only thing that kept him from going off the deep end forever.
This is an endless source of Dramatic Irony, since Akane is always out of the loop, making some scenes with Akane just painful.
Plot-Induced Stupidity: Both straight and justified. A doctor treating TBI (Traumatic Brain Injury) by demanding that everyone play along with a false reality? It speaks volumes that the same doctor frequently smokes in the hospital... near oxygen. On the other hand, there are several points of the story where characters do unbelievably stupid things that are completely consistent with their individual pathologies.
Poor Communication Kills: Haruka's parents and the doctor might have had second thoughts about what they were asking of Takayuki if they knew about his relationship with Mitsuki.
Writer on Board: The doctor's plan to help Harukaby having everyone pretend no time has elapsed might have passed for simple carelessness... if they hadn't done such a painfully-accurate portrayal of the effects of PTSD.